In 3GPP (3rd Generation Partnership Project) which is a project aiming to standardize a mobile communication system, the introduction of Device to Device (D2D) communication is discussed as a new function after Release 12 (see Non Patent Document 1).
In the D2D communication, a plurality of neighboring user terminals (a user terminal group) perform direct communication without passing through a core network. That is, a data path of the D2D communication does not pass through the core network. On the other hand, a data path of normal communication (cellular communication) of a mobile communication system passes through the core network.
Further, in order to perform the D2D communication, a user terminal performs a process of discovering a partner terminal in the D2D communication. The user terminal transmits a discovery-use signal for discovering the partner terminal (or for being discovered by the partner terminal) so as to discover the partner terminal. The user terminal having discovered the partner terminal starts the D2D communication with the partner terminal.